High energy laser systems are called upon to provide a high degree of pointing accuracy control of outgoing high-energy laser light. Some typical applications include directed energy weapons, cutting and welding, optical measurements and surveying, and optical communication, among others.
One impediment toward a realization of a requisite pointing direction control in a practicable embodiment is undesired cavity mirror movement such as could be induced by external vibration and thermal gradients. The movement may result either in a tilting of the cavity mirrors or a displacement thereof in directions generally transverse the cavity axis. The movement of the cavity mirrors effectively displaces the optical axis of the laser and in such a way as to introduce an uncertainty in the pointing direction of the outgoing high-energy laser light.
A further impediment to effective control arises from the thermal absorbtion characteristic of the material of the cavity mirrors. The high-energy laser light is partly absorbed as heat by the cavity mirrors. The heat so distorts the figures of the cavity mirrors as to de-focus the outgoing high-energy laser light.
Movement compensation as heretofore contemplated has included various decoupling mounts for resiliently isolating the cavity mirrors from motion-inducing vibrations. The technique is limited in its effectiveness insofar as the degree of decoupling is never complete. It typically presents considerable high-energy laser system interface difficulties, and is wholly ineffective against movement induced by other than vibrational cases.
The heretofore known thermal compensation techniques for high-energy lasers generally have not been entirely satisfactory. Heat transport off the cavity mirrors by means of a suitable heat transfer fluid requires special sub-systems that often are difficult to integrate into the high-energy laser system, and the physics of the heat transfer process inherently limits its utility for some applications. Another technique is to control the degree of mirror figure distortion by keeping the laser power low enough to avoid the deformations; but it is an inapplicable solution where a high power output is either desirable or is required. So-called open loop control represents a further technique to compensate the thermal distortion. A prediction beforehand of the degree of distortion that is expected with time is projected, and the high-energy laser system, in operation, is compensated on this basis. The utility is often limited here, however, by a fundamental inability to accurately predict how the mirrors will actually deform with a sufficient degree of confidence.